1. Field of the Invention
The present invention relates to inkjet recording apparatuses, and in particular relates to an inkjet recording apparatus in which ink is supplied from a main tank to a sub-tank mounted on a carriage together with a recording head for storing ink to be supplied to the recording head.
2. Description of the Related Art
A serial-type inkjet recording apparatus has been known, in which the recording head is moved in a principal scanning direction so as to form images on a recording sheet while the recording sheet is conveyed by a predetermined distance in a direction perpendicular to the principal scanning direction.
The recording head mounted on a carriage is constructed so as to receive ink from an ink tank in a mounted state on the carriage.
For supplying ink to the recording head, there have been generally known a system (on-carriage system), in which ink is supplied from an ink tank mounted on a carriage together with the recording head, and a system (off-carriage system), in which ink is supplied from an ink tank arranged at a predetermined position other than the carriage to the recording head via a sub-tank mounted on the carriage.
In the on-carriage system, when the kind and amount of ink are increased, the carriage becomes larger in size and heavier in weight due to the increased space for mounting the ink tank.
Furthermore, a drive mechanism for driving the carriage is also scaled up due to the added weight, and a space must be ensured for carriage scanning.
Thus, the on-carriage system has a problem of further increase in size for comparatively large scale recording apparatuses having a large capacity ink tank.
The off-carriage system has an ink supply mechanism including a main tank fixed on the apparatus and the sub-tank moving on the carriage within the apparatus and stably connected to the main tank with a tube.
Japanese Patent Laid-Open No. 2002-234180 (corresponding to U.S. Pat. No. 6,702,433) discloses an ink supply mechanism of the off-carriage system in that the main tank and the sub-tank are stably connected together with a tube.
This patent document illustrates a method for generating negative pressure in the recording head when ink is supplied to the recording head using a difference in hydraulic head between the main tank and the recording head.
Japanese Patent Laid-Open No. 8-300677 discloses that the sub-tank is enclosed from the atmosphere and the negative pressure in the recording head is given by a difference in hydraulic head between the main tank and the recording head in the same way as in Japanese Patent Laid-Open No. 2002-234180.
The off-carriage system may include an ink supply system in that a supply path between the main tank and the sub-tank is disconnected during the movement of the carriage for recording and then the carriage is moved to a predetermined position when necessary so as to connect the main tank to the sub-tank.
In the system disconnecting the supply path between the main tank and the sub-tank during the movement of the carriage, difference in hydraulic head between the main tank and the recording head cannot be used upon generating negative pressure during supplying ink to the recording head. Hence, the negative pressure is generally produced in the sub-tank. For generating the negative pressure in the sub-tank, there are known methods where the negative pressure is produced by increasing the volume of an ink reservoir by a spring force, and the negative pressure is generated by a capillary force of an ink absorber such as a polyurethane foam and a fiber bundle.
In Japanese Patent Laid-Open No. 10-128992, the negative pressure is produced by a spring force. This is comparatively complicated in structure; however, miniaturization is possible.
In Japanese Patent Laid-Open No. 2001-310477 (corresponding to U.S. Pat. No. 6,637,872) and No. 2002-086745 (corresponding to U.S. Pat. No. 6,612,683), methods for generating the negative pressure using an ink absorber are described, and these methods have an advantage of simplicity in structure.
FIG. 5 is a sectional view of a conventional structure using an ink absorber in the sub-tank.
To make the drawing easily understandable, a wide space between an absorber 208 and a sub tank 202 is shown in the drawing; however, in practice the absorber is pressed into contact with the inner wall of the sub-tank with ribs formed on the inner wall of the sub-tank so as to have very small clearance therebetween. The following drawings are the same.
As shown in FIG. 5, the absorber 208 accommodated inside the sub-tank 202 contains ink 213, which is supplied to a recording head 201 via a filter 207.
The sub tank 202 is provided with a porous gas-liquid separation membrane 209 with a water-repellent surface arranged on the absorber 208. The gas-liquid separation membrane is provided with micro pores formed therein, and when a predetermined pressure difference is applied across the thickness of the gas-liquid separation membrane, gas is permeable through the pores while the pores do not transmit liquid.
The sub-tank 202 is provided with a connection part arranged on the side wall for a supply tube 204 for connecting the sub tank to a main tank 203. The supply tube 204 includes a valve 210 which is closed in the period of time other than that for supplying ink from the main tank 203 to the sub tank 202. In such a manner, the sub-tank 202 is hermetically sealed with the valve 210 other than the pores of the gas-liquid separation membrane and the recording head 201.
During ink ejection from the recording head, the negative pressure of the absorber 208 is applied to the recording head so as to enable the recording head to be efficiently supplied with ink from the sub-tank.
Even when a structure which separates the tube off during recording is adopted, a valve is provided in the same way, so that a closed system is secured when the tube is separated.
An evacuation tube 205 from the sub-tank 202 through the gas-liquid separation membrane 209 arranged above the sub-tank 202 is connected to a pump 211, which is driven when ink is supplied from the main tank to the sub tank.
On the other hand, in the period of time other than that for supplying ink, the sub-tank communicates with the atmosphere, so that the pressure in the sub-tank 202 can be maintained at atmospheric pressure through the evacuation tube 205. The evacuation tube 205 may be constructed to be separable from the sub tank, and in this case, the pressure in the sub-tank 202 can be maintained at atmospheric pressure through an open hole of the tube.
For supplying ink from the main tank to the sub tank, a method to pressurize the main tank 203 or a method to change the potential head between the main tank 203 and the sub-tank 202 may also be adopted.
FIGS. 6A to 6C are drawings for illustrating ink behavior within the sub-tank 202, wherein FIG. 6A shows situations in which the ink 213 contained in the absorber 208 in the sub-tank 202 is consumed along with ink ejection from the recording head 201 so that a boundary 214 between the ink and the atmosphere is falling down.
FIG. 6B shows situations in which at the time when residual ink is reduced by the ink consumption mentioned above, the pump 211 is driven so as to supply ink from the main tank 203. That is, gas is aspirated via the evacuation tube 205 and the gas-liquid separation membrane 209 by driving the pump 211, thereby depressurizing the insides of the sub-tank 202.
By the pressure difference due to the depressurization, ink is supplied from the main tank 203 so that the absorber 208 is impregnated with the ink and the boundary 214 comes up. FIG. 6C shows a state that by further supplying ink from the main tank 203, the absorber 208 is filled with the ink 213 so that the boundary 214 reaches the gas-liquid separation membrane 209. Since the gas-liquid separation membrane 209 does not transmit ink at this time, when the ink 213 reaches the entire surface of the gas-liquid separation membrane 209, the entire bottom surface of the gas-liquid separation film 209 is brought into contact with ink, and gas is not aspirated from the upper surface of the gas-liquid separation membrane 209. This terminates the ink supply from the main tank 203 to the sub-tank 202. By repeating the behavior shown in FIGS. 6A to 6C, the state that ink is supplied to the recording head can be maintained.
However, in the system in that the absorber is accommodated within the sub tank so as to generate the negative pressure therein and to supply ink from the main tank to the recording head via the sub-tank, as described with reference to FIGS. 5 and 6A to 6C, when ink is supplied to the recording head, ink may occasionally stop being supplied from the sub-tank.
FIGS. 7A to 7C are drawings illustrating this phenomenon.
FIG. 7A shows a state in which ink contained in the absorber 208 in the sub-tank 202 is consumed along with ink ejected from the recording head 201 so that the start of ink supply is at the time when the boundary 214 fairly falls down. That is, along with ink ejection from the recording head, the boundary 214 between ink and the atmosphere descends. However, the start of ink supply may lag behind the established ink-filling timing in a state in which the boundary 214 further falls down because of consumption measurement error, ink evaporation, and wrong operation. At this time, as shown in FIG. 7A, the ink 213 supplied from the main tank is moved higher than the connection part of the supply tube 204 so as not to sufficiently mix with ink existing in the vicinity of the filter 207 because the boundary 214 falls down, forming individual ink portions.
FIG. 7B shows a state in which ink is further supplied from the main tank from the above state so that the ink reaches the entire gas-liquid separation membrane 209. As shown in FIG. 7B, when the ink portion supplied from the connection part of the supply tube 204 reaches the bottom surface of the gas-liquid separation membrane 209 in a state in which individual ink portions are formed, the entire bottom surface of the gas-liquid separation membrane 209 is covered with ink. Thus, gas in the sub tank cannot pass through the gas-liquid separation membrane 209 from the inside of the sub tank so as to stop further ink supply. As a result, a large portion D not containing the ink 213 remains in the absorber 208. Simultaneously, there are provided an ink portion E bordering on the filter 207 and an ink portion F ranging from the connection part of the supply tube 204 to the gas-liquid separation membrane 209 formed individually. Thus, the two ink portions E and F may come in contact with each other with a small section 215 (FIG. 7B) therebetween, or although not shown, the ink portions E and F may be formed separately from each other. FIG. 7C is a drawing showing a state that after the ink in that state is supplied from the main tank, the ink in the absorber 208 is consumed by the ink ejection from the recording head 201.
In the ink supply to the recording head 201, when ink is supplied to the recording head in the state in which the two ink portions E and F come in contact with each other with a small section therebetween or they are formed separately from each other as described above, in comparison with the state in which the ink portions E and F are continuous as shown in FIG. 6C, the state of FIG. 7C has fewer ink continuous portions, finally resulting in ink discontinuity and ejection failure at the recording head 201.
Such a phenomena is liable to be generated especially when the volume of the ink absorber is large or in a case where ink is difficult to have a continuous state so that the filter area of a portion supplying ink to the recording head is increased.
Thus there has been a problem that the inkjet recording apparatus cannot keep up with the speeding up of the recording head and increase in consumption and size.